This invention relates to an electromagnetic clutch and more particularly to an electromagnetic clutch having an excellent impact strength.
An arrangement of a conventional electromagnetic clutch is shown in FIG. 3. The electromagnetic clutch shown in this figure is employed in a sheet feed mechanism of a copying machine and the like. The electromagnetic clutch comprises a field member 32 in the form of a double-layered cylinder with a bottom surface. The field member 32 has a hollow cylindrical casing 31 extending in an axial direction thereof inside. Inside the field member 32, there is mounted an annular exciting coil 33 (energizing member) on an outer circumference of the casing 31. A first shaft 34 having a through hole 34a extending in an axial direction of the field member 32 is rotatably inserted in the hollow portion of the casing 31.
The first shaft 34 is fixedly mounted with a rotor 36 at one axial end thereof. The rotor 36 is formed with through holes 35 in the form of slot and opposes to the exciting coil 33. A second shaft 37 whose axial length is larger than the first shaft 34 is inserted in the through hole 34a and is fixedly mounted to the first shaft 34 by fastening means, thereby rendering the second shaft 37 rotatable together with the first shaft 34.
A drive force transmitter 38 formed with a gear wheel around an outer periphery thereof is fixedly mounted to the second shaft 37 to be rotatable about the rotary axis of the second shaft 37. The drive force transmitter 38 is concentrically mounted with a contact member 39 with a plate spring (as an elastic member) 40 in the form of a ring interposed therebetween. The contact member 39 is biased to the drive force transmitter 38 by a biasing force of the spring 40, and is disposed opposing to the rotor 36 by a certain distance (clearance).
The thus constructed electromagnetic clutch is operated as follows. Upon being supplied with a power from a drive power source (not shown), the drive force transmitter 38 is rotated together with the contact member 39 around the rotary axis of the second shaft 37. At this time, when a power is supplied to the exciting coil 33 to energize (excite) the same, a flux of magnetic induction is generated around the energized coil 33 and along a magnetic passage defined by the field member 32, first shaft 34 and rotor 36. The rotor 36 attracts the contact member 39 against the biasing force of the spring 40 by leakage magnetic flux passed through the holes 35 of the rotor 36. As a result, the rotor 36 is rotated together with the rotating drive force transmitter 38, thereby rotating the second shaft 37 and the first shaft 34 to which the rotor 36 is fixedly attached.
On the other hand, when the power supply to the exciting coil 33 is suspended, i.e., the exciting coil 33 is deenergized, the magnetic flux disappears, and the magnetic attraction force of the rotor 36 to attract the contact member 39 thereto also disappears. As a result, the contact member 39 is returned to the drive force transmitter 38 by the biasing force of the spring 40, i.e., away from the rotor 36 by the certain clearance. Consequently, even though the drive force transmitter 38 keeps rotating, the rotating force of the transmitter 38 is not transmitted to the first shaft 34 and the second shaft 37, i.e., the rotation of these shafts 34 and 37 is suspended.
The above constructed electromagnetic clutch has the following drawback. There may be the possibility that the drive force transmitter 38 mounted with the contact member 39 is inadvertently dropped on a floor during assembly of the electromagnetic clutch. Also, there may be the possibility that an assembled electromagnetic clutch is inadvertently dropped on a floor during a step of assembling a copying machine (i.e., mounting the electromagnetic clutch to the copying machine).
In such case, it is highly likely that the spring 40 is deformed or bent beyond an elastic limit of the spring 40 owing to a physical impact (stress) applied to the drive force transmitter 38 of the half-way assembled electromagnetic clutch or to the completed assembled electromagnetic clutch when the clutch hits the floor and the like, with the result that part or entire part of the contact member 39, which is mounted to the drive force transmitter 38 with the spring 40 interposed therebetween, is protruded toward the rotor 36. The protrusion (displacement) of the spring toward the rotor side is enhanced by the mass of the contact member. In other words, the greater the mass of the contact member, the larger the displacement of the spring towards rotor side becomes when the impact force is applied to the drive force transmitter. If such an incident occurs, the contact member 39 constantly comes into contact with the rotor 36, thus losing the function as the electromagnetic clutch.